1. Field of the Invention
The present invention relates to a solid-state image pickup element and a method of producing it, and more particularly to formation of an intralayer lens.
2. Description of the Related Art
Recently, in solid-state image pickup elements, increase of the number of imaging pixels is advanced to the level of gigapixels or more, and miniaturization of an element is going on progressing. Under such circumstances, the sensitivity is requested to be improved. However, an on-chip lens which is formed on a color filter and in the uppermost layer is not sufficient for improving the sensitivity. A technique has been proposed in which a lens (intralayer lens) is further formed between an on-chip lens and a sensor portion, thereby enhancing the light converging function. As an example of such an intralayer lens, there is a convex superior portion which is formed on a face planarized by filling a step caused by a transfer electrode or the like, and in which the surface side is formed as a convex face.
As an example of a method of producing such a convex superior portion, there is a method in which a stacked layer of a film of a lens material for the convex superior portion and a resist is dry etched. In the method, a film of the lens material is deposited, the resist on the film is patterned into a shape of the lens, and the lens shape of the resist is transferred to the lens material film by dry etching.
Another method of enhancing the sensitivity by adjusting the focal length of a lens to a desired length in a wide range has been proposed (JP-A-2000-164837).
In this method, a resist mask having a shape of a lens is formed on a layer of a material for an intralayer lens, the lens shape of the resist mask is transferred to the layer of the intralayer lens material by the etch back method, thereby forming an intralayer lens, and the focal length is adjusted by the etch back amount.
When an intralayer lens is disposed below an on-chip lens formed by a microlens as described above, incident light is converged in two steps to enable a larger amount of light to enter a light receiving portion. The sensitivity of a solid-state image pickup element can be enhanced as compared with the case where only an on-chip lens is formed.
In this way, the height of an intralayer lens of a solid-state image pickup element can adjust the focal length by means of controlling the etch back amount. The intralayer lens is formed by a method which is performed after formation of metal wiring, and in which the lens material is used also as a wiring protective film (passivation film). In a step of forming the lens, when the lens thickness is reduced in the process of etching back the lens material, also the passivation film for a wiring portion is thinned, and the wiring portion may be exposed. Therefore, the film thickness has limitations, and reduction of the height of the intralayer lens is restricted. Also when the lens material is not used also as the wiring protective film, the irregularity of the surface of the wiring portion may cause the accuracy of a step of patterning the lens or the like to be reduced.